The present invention relates to a planar inductance element such as a planar inductor or a planar transformer, either having a ferromagnetic film.
In recent years, many researches and developments have been conducted for miniaturization of various electronic apparatuses. However, miniaturization of the power supply is far less advanced than the miniaturization of the main electronic section. As a consequence, the volume ratio of the power supply to the main electronic section is ever increasing. The miniaturization of an electronic apparatus largely depends upon the realization of LSI of the various circuits incorporated in the apparatus. Magnetic components required in the power supply, such as inductors and transformers, have yet to be miniaturized or made in the form of LSIs. The magnetic components are mainly responsible for the increase in the volume ratio of the power supply to the main electronic section.
In order to reduce the volume ratio, it has been proposed that planar magnetic elements be used. A planar magnetic element is a combination of a planar coil and a ferromagnetic layer. Studies have been conducted to improve the performance of planar magnetic elements. Ferromagnetic film for use in planer magnetic elements need to meet two requirements. First, it must have low loss in a high-frequency region of 100 kHz or more. Second, it must accomplish high saturation magnetization.
The permeability the ferromagnetic film exhibits in the high-frequency region results mainly from rotation magnetization process. To accomplish an ideal rotation magnetization process, high-frequency magnetic excitation must be conducted on the ferromagnetic film, along the magnetic hard axis of the film which exhibits uniform inplane uniaxial magnetic anisotropy. Physical parameters of importance which the ferromagnetic film must have are permeability along the magnetic hard axis and coercive force.
The high-frequency permeability has complex relation with the other various physical parameters. The physical parameter which more correlates with the high-frequency permeability than any other parameter may be the magnetic anisotropy field. The high-frequency permeability is substantially proportional to the reciprocal of the magnetic anisotropy field. In a magnetic element such as a thin-film inductor, the optimum value for the permeability of the ferromagnetic film changes with design. To exhibit high permeability in a high-frequency region so as to be used in a thin-film inductor, the ferromagnetic film must have inplane uniaxial anisotropy and controllability of the magnetic anisotropy field.
The greater the saturation magnetization of the ferromagnetic film of a magnetic element such as a thin-film inductor, the greater the possibility for the available power range and saturation current to increase. High saturation magnetization property is therefore important to ferromagnetic film for use in magnetic elements such as thin-film inductors.
Needless to say, ferromagnetic film which has low loss in the high-frequency region and which achieves high saturation magnetization is promising material for use in thin-film magnetic heads. This is because the recording density, the coercive force of recording media, the magnetic energy product, and the operating frequency of the heads are increasing.
Ferromagnetic film must meet the requirements described above, so that it may be used in any other magnetic element. When ferromagnetic film is employed in the form of either a single layer or a stacked layer in a magnetic element such as a thin-film inductor, its property somewhat deviates from the intrinsic property. This is a problem concerning the use of ferromagnetic film.
In the thin-film process technology, layers and films are formed, in most cases, on a substrate wafer before a number of magnetic elements are formed on the substrate. Among these layers and films are a resin layer, insulator film, metal film, protection film, and a patterning mask layer. After the magnetic elements, each having at least one ferromagnetic film, have been formed, films and layers are formed one on another, over the magnetic elements, in most cases.
These layers and films are formed by performing a sequence of processes including physical vapor deposition (PVD), chemical vapor deposition (CVD), plating, spin coating, and baking (e.g., curing), and the like. In the PVD or CVD, the temperature of the unfinished product inevitably rises to some extent while the layers and films are being formed, even if the substrate temperature is not raised during the PVD or CVD.
The ferromagnetic film and the other films and layers must be subjected to patterning to form electrodes and to form slits and through holes in the films and to achieve device isolation, thereby to provide magnetic circuits. The patterning is performed by forming masks in various methods and then by dry- or wet-etching the layers and films by using the masks.
In most cases, the stress applied to the surface of the ferromagnetic film or to the interface between the ferromagnetic film and any other film changes during the patterning. The stress applied to one part of the ferromagnetic film differs from the stress applied to any other part, in both magnitude and direction. In other words, the stress is dispersed in magnitude and direction. Further, the internal stress in the ferromagnetic film may change when the stress is released as the heat treatment proceeds. Changes in the stress and stress dispersion greatly influence the magnetic properties of the ferromagnetic film. The changes appear the most notable problem, in view of the desired magnetic properties to impart to the ferromagnetic film, the control of these properties, the coercive force relating to the rotation magnetization process, and the necessity of maintaining soft magnetism for achieving low loss.
The magnetic anisotropy of a ferromagnetic film responses to a strain in the film which has resulted from the stress applied to the film. An anisotropic stress, if any, applied to the ferromagnetic film induces magnetic anisotropy. The energy of the magnetic anisotropy thus induced is proportional to the magnetostriction constant of the ferromagnetic film.
To cause the ferromagnetic film incorporated in a planar inductance element to exhibit the designed magnetic properties, two alternative methods can be used. The first method is to reduce the anisotropic stress applied on the film during the manufacture of the inductance element. The second method is positively to utilize the anisotropic stress applied during the through-process of the inductance element, thereby to control the magnetic anisotropy of the ferromagnetic film. The inventor hereof have studied both methods to find that high technology is required to perform the second method successfully. It appears extremely difficult to use the second method to control the magnetic properties of the ferromagnetic film.
Generally, the inductance of a planar inductance element is almost proportional to a change in the magnetic anisotropy field of the ferromagnetic film used in the inductance element. In other words, the inductance is substantially proportional to a change in the high-frequency permeability. This change resulting from the magnetic excitation which is undergoing along the magnetic hard axis of the film during the rotation magnetization process.
Hence, the properties of the planar inductance element, such as inductance L and Q factor corresponding to the loss of inductance, will change if an anisotropic stress is generated or the dispersion of this stress takes place while the inductance element is being manufactured, being molded, or being assembled into a miniaturized power supply. This causes the deviation of the inductor properties from the typical values of the lot or the scattering of the specific values of the lot. Consequently, the planar inductance element fails to fulfill specification, and its manufacturing cost will increase.
The above-mentioned problems inherent in the planar inductance element have been found by studying the process of manufacturing the element. In view of the properties of the ferromagnetic film, it is important for the film to have a small magnetostriction constant. If the magnetostriction constant is small, the stress on the film will not influence the permeability, etc. of the film.
Various ferromagnetic materials are known to achieve high saturation magnetization and exhibit sufficient inplane uniaxial magnetic anisotropy. They are therefore suitable as materials of ferromagnetic films for use in planer inductance elements. Quite a few of these materials have a large magnetostriction constant. To reduce the magnetostriction constant, such a material may be doped with a specific amount of Si or the like. If the material is doped with Si or the like, however, the spontaneous magnetization will decrease inevitably. Consequently, ferromagnetic materials which need to attain high saturation magnetization of 1.5 T or more, except those having specific compositions, can hardly acquire a magnetostriction constant of zero.
Some measures must therefore be taken in the planar inductance element in order to minimize the changes of effective permeability which result from the magnetoelastic energy effect. More precisely, the planar inductance element must have a ferromagnetic film which is so designed as to prevent, as much as possible, the inductance of the element from changing as the high-frequency permeability changes. The term "effective permeability" means the permeability which contributes to the inductance value, pertaining to the high-frequency magnetic excitation undergoing in the planar inductance element.
As described above, it has been demanded that a ferromagnetic film be provided which maintain high saturation magnetization, soft magnetism and excellent high-frequency permeability, whose effective high-frequency permeability changes only a little despite the magnetoelastic energy effect during the manufacture of the miniaturized magnetic element having the ferromagnetic film, and whose effective permeability is hardly influenced by the process of manufacturing the magnetic element. It has also been demanded that a miniaturized inductance element be provided which incorporates an inductance element having such a ferromagnetic film.
As indicated above, the conventional planar inductance element is disadvantageous in that during the manufacture of the element, the magnetic properties of the ferromagnetic film changes because the stress applied to one part of the film differs the stress applied to another part of the film, in both magnitude and direction.
In view of the foregoing, the present invention has been made. Its object is to provide a ferromagnetic film whose effective high-frequency permeability changes or deteriorates but a little, despite the difference between the stresses applied to the different part of the film, thereby to improve the productivity and yield, and also to provide a planar inductance element having the ferromagnetic film.